32 IMPORTANCE OF CLASSIFICATION. 



vessels and their valves, of the air-vessels and the lungs, 

 and of the mechanical process of breathing, will compel 

 your reference to hydraulics and pneumatics ; while, to 

 understand how it happens that between the successive 

 expirations and inspirations there always occurs an 

 interval which allows time for the two noxious fluids 

 nitrogen gas and carbonic acid gas which are ejected 

 at the same instant, to separate, the first to ascend as 

 the other descends, the supply of a current of uncon- 

 taminated atmospheric air being meanwhile provided 

 for ; and again, to seize the rationale of that striking 

 arrangement in vegetable physiology, in consequence of 

 which what is hurtful to man becomes beneficial to 

 vegetables, at the same time that the oxygen which 

 vegetables do not need is separated by them in its 

 utmost purity for the use of man, you must appeal for 

 aid in part to chemistry, in part to pneumatics. 



Pursuing such trains of inquiry, you will soon feel 

 the importance of aiming to estimate the real and the 

 relative value of the various sciences, as well as of 

 tracing their mutual connexion and dependence; and 

 hence feel, also, how desirable it is to arrange the 

 objects of human thought in grand divisions. Whatever 

 be the general classification which you adopt, carefully 

 assign to each grand division its appropriate portion, 

 weigh well its natural subdivisions, and assign to each 

 of them its appropriate proportions also. Let the 

 attempts at classification be, at first, entirely and 

 exclusively your own ; . and after you have formed 

 three or four such schemes, subject them to the inspec- 

 tion of some one who is experienced in matters of 

 arrangement. From his suggestions and modifications 

 you will gain a useful lesson, by learning the nature, the 



